Heretofore, in the field of gasoline engines, a combustion mode where an air-fuel mixture is forcibly ignited by a spark discharge from a spark plug (spark ignition (SI) combustion) has been commonly employed. Recent years, researches for applying so-called “homogeneous-charge compression ignition (HCCI) combustion” to gasoline engines in place of the SI combustion have been conducted. The HCCI combustion is intended to auto-ignite an air-fuel mixture formed in a cylinder (combustion chamber) of an engine without relying on spark ignition, under a high-temperature/high-pressure environment created by compressing the air-fuel mixture by a piston. The HCCI combustion is a combustion mode in which auto-ignition occurs simultaneously at many positions in the cylinder, and it is assumed that a combustion period thereof becomes shorter and thereby higher thermal efficiency is obtained, as compared with the SI combustion.
In this connection, for example, JP 2007-132319A (hereinafter referred to as “Patent Document 1”) proposes a control system for an HCCI engine, which is designed to open an exhaust valve during an intake stroke as well as during an exhaust stroke to allow high-temperature burned gas discharged once to flow back into a cylinder from an exhaust port so as to raise an in-cylinder temperature to promote auto-ignition of an air-fuel mixture.
Based on opening an exhaust valve during an intake stroke to introduce high-temperature burned gas into a cylinder, as in the Patent Document 1, auto-ignition of an air-fuel mixture can be effectively promoted to reliably induce HCCI combustion, particularly, in a low engine load range where an air-fuel mixture is less likely to be auto-ignited. However, if the operation of introducing the burned gas into the cylinder is continuously performed through an engine operating region where an engine load is relatively high (i.e., a fuel injection amount is relatively large), the auto-ignition of the air-fuel mixture is liable to be excessively promoted, which causes abnormal combustion such as preignition.